Regional differences in zooplankton-associated bacterial communities and aquaculture pathogens across two shelf seas

Microbial organisms can persist in the marine environment in a number of ways, for example during a free-living life stage or colonising the surface of plankton. These microorganisms include pathogens which may adversely impact the aquaculture and shellfishery sectors. Microbial diversity is well documented in nearshore bivalve culture environments, however, less is known about microbial communities in offshore environments. Additionally, oceanic fronts and currents may facilitate (or inhibit) planktonic dispersal, which can then allow for the movement of associated microbes across otherwise restrictive barriers. This study used zooplankton samples collected from the Irish and Celtic Seas to assess the zooplankton community structure and that of its associated microbial community. Zooplankton samples were identified using microscopy and screened using polymerase chain reaction (PCR) to identify pathogens with a potential aquaculture impact. Samples from four subareas representing the Irish Sea, Celtic Sea Front, Eastern Celtic Sea and Southern Celtic Sea also underwent sequencing of the bacterial V3–V4 region to define the bacteriome within these areas. All samples were dominated by crustaceans, and high molluscan numbers were evident around the Celtic Sea Front. There were distinct bacterial profiles, dominated by Cyanobacteria and Proteobacteria, and a positive relationship between Proteobacteria and copepods, chaetognaths and molluscs. A degree of connectivity was observed between the Eastern Celtic Sea and Celtic Sea Front, but the front acted as a barrier to the Irish Sea. The study also detected the bacterium Vibrio splendidus, potentially pathogenic to bivalves, and a low prevalence of a haplosporidian sequence detected in just one previous study to date.